THREE DIMENSIONAL THERMOACOUSTIC DEVICE COMPOSED OF NA- NOPOROUS MATERIAL AND THE METHOD TO FABRICATE SUCH A DEVICE

Document Type and Number:

WIPO Patent Application WO/2018/026267

Kind Code:

A1

Abstract:

The present invention is in the field of a three dimensional thermoacoustic device composed of nanoporous material and the method to fabricate such a device, as well specific uses of said device, which device may have an improved sound pressure level (SPL), a flat frequency response, and a high thermoacoustic efficiency.

etching openings (71) in the top conducting layer, and removing dielectric material (22) to create freed space (61) in between the nanoporous material therewith providing at least one 3D-structure .

2. Method according to claim 1, further providing a sealing layer (84) on the top conducting layer.

3. Method according to any of the preceding claims, wherein at least one of the openings (71) are symmetrical, the second dielectric layer (22) is TEOS, and the vertically aligned electrically conductive nanoporous material (41) are carbon nanotubes (CNT) .

4. Method according to any of the preceding claims, wherein at least one of the nanoporous material (41) has an electrical conductance of >103 σ (S/m) at 20 °C, and the nanoporous material (41) has a specific heat of 300-900 J/kgK.

5. Method according to claim 4, wherein at least one of the bottom conducting layer (31) and top conducting layer (32) is provided with patterns.

6. Method according to any of the preceding claims, further comprising providing a conductive supporting layer (23) on the bottom conducting layer (31) .

7. Method according to any of the preceding claims, further comprising providing a sacrificial layer (81) .

8. Method according to any of the preceding claims, provid- ing the sacrificial layer (81) on the conductive supporting layer (23 ) .

9. Method according to any of the preceding claims, wherein at least one of

the substrate is provided with a back side dielectric layer (28),

the second dielectric layer is partly applies, such as by dep¬ osition, on the first dielectric layer,

the second dielectric layer is provided with a thermally and chemically stable masking layer (83) ,

the nanoporous material is provided with at least one confor- mal layer, such as an dielectric layer, and

the bottom conducting layer is provided with a protecting layer (82) .

10. Method according to any of the preceding claims, wherein the substrate is provided with a cavity (11).

11. Method according to any of the preceding claims, wherein at least one of the nanoporous material (41) , openings (71), receiving space (51), top conducting layer (32), and bottom conducting layer (31) is patterned using lithography and/or e-beam.

(ic) at least one top electrical conductive element (32) adapted for interconnection,

(Id) an electrical input (37), and

(le) an electrical output (38),

wherein the electrical input (37), at least one top electrical conducting element (32), at least one 3D-nanoporous material element (42), at least one bottom electrical conducting element (31), and electrical output (38) provide at least one electrically conductive path, and

(ii) an electrical power supply (90) for driving the at least one sandwiched structure by providing power thereto.

13. Device according to claim 12, obtainable by a method according to any of claims 1-11.

14. Device according to claim 12 or 13, wherein the nanopo- rous material has a ratio of surface area SA (m2) to volume V (m3) of > 104 nr1.

15. Device according to any of claims 12-14, wherein the nanoporous material is selected from carbon comprising material -

16. Device according to claims 15, wherein the nanoporous material is carbon nanotubes (CNT) or graphene.

17. Device according to any of claims 12-16, wherein at least one top electrical conductive element (32) has at least one array of symmetrical openings.

18. Device according to any of claims 12-17, wherein at least one interconnected series of n*m sandwiched structures are provided, wherein n and m are each independently ne [1, 106] , and m e [1, 106] .

19. Product comprising a device according to any of claims 12-18, wherein the product is preferably selected from a sensor, a gas sensor, an oxygen sensor, an actuator, a MEMS, a micro reactor, a micro evaporator, a micro thruster, a nano- filtration device, a microphone, a microheater, and a capaci¬ tor .

20. Use of a device according to any of claims 12-18 or product according to claim 19, the device comprising (i) a vertical 3D thermoacoustic heater (200) comprising at least one sandwiched structure (300), the sandwiched structure comprising (iO) a substrate (10), (ia) at least one bottom electrical conducting element (31) adapted for interconnection, (ibl) an electrical conductive intermediate layer (41) com- prising vertically aligned nanoporous material, (lb2) in said intermediate layer freed space (61) , therewith forming at least one 3D-nanoporous material element (42), and (ic) at least one top electrical conductive element (32) adapted for interconnection, (Id) an electrical input (37) , and (le) an electrical output (38), wherein the electrical input (37), at least one top electrical conducting element (32), at least one 3D-nanoporous material element (42), at least one bottom electrical conducting element (31), and electrical output (38) provide at least one electrically conductive path, and (ii) an electrical power supply (90) for driving the at least one sandwiched structure by providing power thereto for providing at least one of a sound pressure level (SPL) of at least 37 dB normalized to a 1W input power at 3kHz at a distance of 3 cm, a flat frequency response, and a high thermoacoustic effi- ciency.

Description:

Title Three dimensional thermoacoustic device composed of na ¬ noporous material and the method to fabricate such a device

FIELD OF THE INVENTION

The present invention is in the field of a three dimen- sional thermoacoustic device composed of nanoporous material and the method to fabricate such a device.

BACKGROUND OF THE INVENTION

In a historical perspective for sound producing devices horns occupied the audio market for more than 40 years until a good performance standard electrodynamic loudspeaker was first built in the 1920' s. In a next generation electrostatic loudspeakers were invented in the 1960's, which had a better sound performance. More recent a so-called distributed mode loud ¬ speaker is designed in 1990' s. Figure 1 shows this historical development of commercial speakers. Details of this figure can be found on the internet, such as edisontechcenter, audiohol- ics, and upv.es. Electrodynamic loudspeakers are widely used nowadays in various audio systems, sound boxes and headphones, and, after replacing the horns, have ruled the market for more than 90 years. Other types of design, such as electrostatic loudspeakers, flat panel electrostatic loudspeakers, and distributed mode loudspeakers (DML) joined the market afterwards, but never surpassed the electrodynamic speakers in terms of numbers. Over all these years improvements were only made in terms of design for these electrodynamic loudspeakers, and not in terms of physics behind the speakers. An electrodynamic speaker still consists of a movable diaphragm and a magnet coil assembly which is driven by an electric source to create vibrations/expansions in the air which are perceived as sound by the human ear.

In an alternative approach in 1917, a first thermoacoustic device to serve as a loudspeaker was designed by H. D. Arnold and B. Crandall. The thermo-phone, as they called it, was formed mainly by a 0.7 μιη thin platinum strip and was driven by an alternating electric current. Later a combination of thermoacoustic sound emission with micro-fabrication and a novel porous silicon material was made. Varieties of thermoacoustic loudspeaker designs emerged after 2008, including designs with Al suspended wires, Au nanowires, silver meshes, graphene, reduced graphene oxide (RGO) , etc.

Comparing with conventional electrodynamic loudspeakers a use of the thermoacoustic effect for sound generation has clear advantages. It is noted that inside a thermoacoustic sound source there is no moving diaphragm. Diaphragms are typ ¬ ically considered to be fragile, so in contrast a thermoacoustic device would have an improved robustness. For the thermo ¬ acoustic device the sound wave is not produced by mechanical movement, no voice coil nor magnet is needed for that purpose. Thus thermoacoustic loudspeakers and the like have a simple structure, can be free of a magnet, require no assembly after fabrication, and production processes thereof are in principle at least to some extent compatible with semiconductor pro ¬ cesses. Recent development of micro-fabrication and materials have sparkled a renewed interest in the thermoacoustic loudspeaker .

Prior art thermoacoustic designs focus on employing planar thin film heaters, in which the electrical current is parallel to a substrate thereof. As a consequence of the planar design 50% of the air expansion is directed towards substrate which is considered to limit the thermoacoustic performance e.g. in terms of efficiency reflected in emission of sound/pressure waves. A schematic of a planar 2D heater is shown in Figure 3A. Examples of such devices can be found in Wei, et al. in "Ice-Assisted Transfer of Carbon Nanotube Arrays.", Nano Letters, 2015. 15(3): p. 1843-1848, Xiao, et al . in Nano Lett., 2008. 8(12): p. 4539, and Xiao, et al. in "High frequency response of carbon nanotube thin film speaker in gases.", J. Applied Physics, 2011. 110(8) : p. 084311.

The frequency response of these 2D planar thin film heating designs is insufficient at low frequencies and excessive at high frequencies, which is found to degrade the fidelity of sound. Moreover, the prior art of thermoacoustic devices require fabrication approaches that are not compatible with standard cleanroom fabrication technology. For example, the fabrication of CNT thin film or thin yarn requires spinning of the CNT fibers, assembly and transfer steps, which steps are not compatible with automated cleanroom fabrication steps and not scalable for mass production. In addition the prior art metal film based thermoacoustic sound sources have a low sound pressure level (SPL) . Also all existing CNT, metal film and wire designs are fragile and occupy a large chip area.

The present invention relates to a thermoacoustic device composed of nanoporous material and a method to fabricate such a device, which overcomes one or more of the above disad ¬ vantages, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates in a first aspect to a method of producing a three dimensional thermoacoustic device 100 according to claim 1. The present method is found to be compatible with mass production processes; a produc- tion capacity of 148 devices per 4 inch wafer was already achieved. The present device is improved e.g. in terms of a more efficient exchange of heat with ambient air due to the use of vertically aligned nanoporous material, which provides a large surface area to volume ratio; the term "verti- cally" in this respect relates to (in a vertical direction) aligned nanoporous material which at least partially extends in a vertical direction, such as from 10-170 degrees with respect to a (surface of the) substrate (slightly upwards), and preferably from 20-160 degrees, more preferably from 30- 150 degrees, even more preferably from 45-135 degrees, such as from 50-130 degrees or from 60-120 degrees, and typically from 70-110 degrees, such as from 80-100 degrees, such as 90 degrees (perpendicular to the substrate and hence fully vertical) ; an ability to lithographically pattern the nanopo- rous material in 3D microstructures providing a tremendous design freedom; a control of a direction of the pressure wave using different geometrical designs or using integrated speaker arrays is possible; the speaker arrays are capable of operating separately, sequentially, in phase-shift mode, in parallel mode, in frequency scan mode, in spatial scan mode, in spatial distribution mode, in intensity mode, in pulsed mode, variations thereof, and combinations thereof. In addition it is also possible to use at least one frequency, such as harmonic frequencies; a flat frequency response in the range of 1kHz to 20kHz is produced, which is now considered possible due to an absence of mechanical mov ¬ ing components which suffer from (mechanical) resonance modes; it provides better sound fidelity as compared to existing speakers; a more equalized frequency response over the range of 1kHz to 20kHz which is considered due to the 3D microstructure and high surface area to volume ratio; a scalable and suitable method for ma s s production is p ro - vided.

In the method a substrate 10 is provided, such as made of a ( semi- ) conducting metal, glass, a ceramics, preferably Si, such as in the form of a wafer. On the substrate a first dielectric layer 21 is applies, such as by deposition, , such as a thermal oxide layer, a LPCVD layer, or a PECVD layer; the dielectric layer provides thermal isolation and electrical isolation; a typical thickness of the layer 21 is 0.1-5 pm, preferably 0.2-3 μπι, more preferably 0.5-2 μκι, such as 1.0- 1.53 um; the layer is thick enough to provide thermal and electrical isolation, but need not be too thick. On the dielectric layer at least one bottom conducting layer 31 is applies, such as by deposition,; in view of further process steps, such as formation of the nanoporous material, the bottom conducting layer is made of material with a high enough melt temperature, such as above 700 K (427 °C) , preferably above 750 K (477 °C) , such as above 800 K {527 °C) . Suitable materials are Ti, W, Cu, Au, preferably Ti and Cu, whereas Al is considered less suited in view of its melting point. Some materials are provided with a protective layer (e.g. 82) in view of e.g. further etching (see e.g. fig. 5a) . Optionally a sealing layer 84 is provided on the top conducting layer, e.g. to protect the layer in view of patterning and especially in view of etching, such as a 0.05 μκι to Ιμηι SiC layer by PECVD (see fig. 5a). The conducting layer may be deposited using PVD, PECVD, sputtering, and the like; a typical thickness of the layer 31 is 0.1-5 μπι, preferably 0.2-3 um, more preferably 0.5-2 pm, such as 1.0-1.5 μη; the layer is thick enough to provide good electrical conductance, but need not be too thick, on the bottom conducting layer a second dielectric layer 22 is applies, such as by deposition, , such as a TetraE- thyl OrthoSilicate (Si (OC2H5) 4 ) (TEOS) layer; this second die ¬ lectric layer functions partly as a sacrificial layer as most of it is removed such as etched away in further process steps; the second dielectric layer 22 may be provided by spinning or PECVD; the layer 22 is relatively thick, such as 1-50 μιη, preferably 2-30 μιη, more preferably 5-20 μπι, such as 10-15 μπι; the thickness of the layer is considered to depend largely on the nanostructures provided in later process steps. The die- lectric layer is then provided with at least one receiving space; thereto a lithographic step, such as an I-line step, if performed, in order to provided well defined microscopic/nano- scopic receiving spaces; these receiving spaces typically have a well-defined geometry, e.g. in view of a height and cross- section thereof; the second dielectric layer is etched through towards the bottom contact layer; for etching a dry etch process may be used; an example of such a dry etch process is DRIE. In the at least one receiving space vertically aligned electrically conductive nanoporous material 41 is provided; in view of further process steps the material is nanoporous, i.e. has at least some porosity wherein porous volumes are in the microscopic and/or nanoscopic scale; the porous volume is typically from 10-98% of the total volume of the receiving space, preferably 20-95%, more preferably 30-90%, such as 50-75%;

more importantly, the present method provides accurate control of the porosity of the porous material, such as by providing thin film (nanoscale) conformal coatings, such as by ALD and LPCVD. In view of the thermoacoustic properties of the device the present nanoporous material is also electrically conduc- tive as well as thermally conductive; as special feature is that the nanoporous material is aligned in a vertical direction, i.e. typically perpendicular to the substrate; the nanoporous material is grown in electrical/thermal contact with the bottom conducting layer; in an example the nanoporous material relates to carbon nanotubes which may be grown using an Fe-catalyst, and typically at a temperature of about 825 K (-550 °C) . In between a first receiving space comprising ver- tically aligned nanoporous material and a second receiving space comprising vertically aligned nanoporous material some dielectric material 22 is (still) present. This dielectric ma ¬ terial is in a later stage etched away, such as by using a dry etching procedure which is found to prevent damaging of the porous material; also in view thereof the vertically aligned material is nanoporous and therewith allows gas species which are capable of etching to pass through the nanoporous material. On the nanoporous material and remaining dielectric ma ¬ terial 22 a top conducting layer 32 is applies, such as by deposition, , such as by sputtering, by PVD, and the like; the top conducting layer is in electrical and thermal contact with the nanoporous material; a typical thickness of the layer 32 is 0.1-10 μπι, preferably 0.2-5 μπι, more preferably 0.5-4 μπι, such as 1.0-2.5 μιη; the layer is thick enough to provide good electrical and thermal conductance and a low thermal capacity as well as to provide a good {conformal} sealing of the nanoporous material underneath, but need not be too thick; suitable materials are Ti, W, Cu, Au, Al, Ni, preferably Ti, Al and Cu; some may have pin holes in the top layer, which can cause bubbling in photoresist in a later stage of lithography; therefore, optionally an extra layer is applied to seal the pin holes, this extra layer can be removed easily; an example is a 0.5-5 ym Si0 2 layer, e.g. TEOS . The top conducting layer is then patterned, typically using alignment markers in order to have a further pattern coincide with an earlier pattern; the pattern provides openings in the top metal layer wherein the openings and remaining dielectric material 22 coincide, possible apart from some dielectric material in between adjacent and closely packed vertically aligned nanoporous materi- als 42; here it is noted that the receiving spaces, and as a consequence vertically aligned nanoporous material, may be distributed evenly (in an horizontal plane) over/in the dielectric material 22, or may be distributed unevenly (see e.g. fig. 3b) such as in groups of two; the top conducting layer may be etched by plasma dry etching, such as by CI2 or HBr plasma, typically at a low pressure (133-1330 Pa, (1-10 Torr, e.g. 5 Torr}); in a further step the dielectric material 22 is removed, such as by etching with HF vapor, typically at a low pressure (133-1330 Pa, (1-10 Torr)), to create freed space 61 in between the nanoporous material without damaging said nanoporous material (the present nanoporous material, such as CNT, was analyzed with Raman spectroscopy before and after be ¬ ing exposed to HF vapor and it showed no deterioration in material quality, such as crystal quality) , therewith providing at least one 3D-structure; at the same time the top conducting layer interconnects adjacent vertically aligned nanoporous ma ¬ terial. A horizontal distance between freed spaces is from 100 nm upwards to 1 mm. A distance between adjacent c.q. individual carbon nanotubes is found to be about 20-50 nm; typically at least a few CNT' s are present in a free space. By varying the size of the freed space and/or space occupied by the nanoporous material and/or dimensions of the nanoporous material

(width/height) characteristics of the thermoacoustic device may be tuned; for instance for a certain application frequency

(single frequency or a band of frequencies ) an optimized performance can be achieved by tuning size and shape of the opening; likewise a volume may tuned, and thereby e.g. the heat capacity. The top conducting layer may further be provided with a masking layer underneath, such as SiC or SiN with a thickness of 0.2-5 urn. This masking layer can be used to reduce the gap width between the nanoporous material and second sacrificial dielectric layer.

In a second aspect the present invention provides a three dimensional thermoacoustic device 100 according to claim 10. The thermoacoustic device is considered to utilize the 3D nanoporous microstructures to produce sound; the thermoacoustic device is considered to convert electrical energy into thermal energy which in turn is converted into acoustic waves, and these waves are perceived as sound by the human ear. The thermoacoustic device is a sandwiched structure composed of a bottom interconnecting layer, a middle layer comprising nanoporous material and a top conductive layer. Both the top, bottom and moreover, the middle nanoporous layers may be used to exchange energy with the environment. When the energy is in the form of heat the device is considered to function as a thermoacoustic transducer. Therein (i) a vertical 3D thermo- acoustic heater 200 is present, the heater comprising at least one sandwiched structure 300, such as one being formed by the above method, wherein the sandwiched structure comprises a substrate 10, at least one bottom electrical conducting element 31 adapted for interconnection, an electrical conductive intermediate layer 41 comprising vertically aligned nanoporous material, in said intermediate layer freed space 61, therewith forming at least one 3D-nanoporous material element 42, and at least one top electrical conductive element 32 adapted for interconnection, an electrical input 37 for providing an electrical current, and an electrical output 38; the electrical input 37, at least one top electrical conducting element 32, wherein the top conducting element may be formed of a metal or of the conductive nanoporous material wherein two adjacent vertically aligned nanoporous materials may form a continuous structure, at least one 3D-nanoporous material element 42, at least one bottom electrical conducting element 31, and electrical output 38 provide at least one electrically conductive path; and an electrical power supply 90, typically an AC supply, for driving the at least one sandwiched structure by providing power thereto; typically the power supply is oper- ated at a frequency of 10 Hz-500 kHz, preferably 15 Hz-300 kHz, more preferably 20 Hz-100 kHz, such as 40 Hz-20 kHz, i.e. in a sound range that can typically be perceived by human beings, optionally including at least one harmonic series thereof; it is noted that in principle also higher frequen- cies, such as ultrasound frequencies, e.g. 300 kHz-lO Hz, such as 1-5 MHz, could be generated, such as for other applications; the power supply typically uses a current of 0.1 mA-lA, preferably 1-500 mA, more preferably 10-250 mA, such as 50-100 mA, a low voltage of 0.01-10 V, preferably 0.1-5 V, more pref- erably 0.5-3 V, such as 1-2 V; it is found that in view of the present layout a frequency as provided is typically doubled; such may be "corrected" e.g. by providing a DC bias to remove said doubling. In a third aspect the present invention relates to a product comprising the present device , such as a sensor, such as a gas sensor, such as an oxygen sensor, an actuator, a MEMS, a micro reactor, a micro evaporator, a micro thruster, in a nan- ofiltration device, in a microphone, in a microheater, and in a capacitor. For instance, in a microheater a much better control of bubble generation is achieved, as bubbles in the present device are of a size equal to or smaller than a distance of two ad acent nanoporous structures; the bubbles typically disturb micro-heating significantly.

In a fourth aspect the present invention relates to a use of the present device for providing at least one of a sound pressure level (SPL) of 37 dB or more normalized to a 1W input power at 3kHz at a distance of 3 cm, a flat frequency re- sponse, and a high thermoacoustic efficiency.

It is noted that some of the steps may be performed in a different sequence, and/or at a later or earlier stage.

Thereby the present invention provides a solution to one or more of the above mentioned problems.

Advantages of the present invention are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to a method according to claim 1, with e.g. reference to figs. 3 and 5.

In an exemplary embodiment of the present method the openings 71 are symmetrical, such as hexagonal, circular, ellipsoidal, square, rectangular, triangular, octagonal, or multigonal, preferably hexagonal. The hexagonal openings are found to provide a more mechanically stable device. At an edge of the present device the openings, and likewise vertically aligned nanoporous material, may form part of said symmetrical element, such as a half thereof.

In an exemplary embodiment of the present method the nanoporous material 41 has an electrical conductance σ of >10 3 (S/m) at 20 °C, preferably >10 4 (S/m) , more preferably >10 5 (S/m); even more preferably >10 6 (S/m) , such as >10 7 (S/m) (according to ISO 14237) . The nanoporous material 41 preferably has a specific heat of 300-900 J/kgK, more preferably 400-800 J/kgK, such as 500-750 J/kgK; the specific heat is preferably not too high and not too low in view of thermoacoustic proper ¬ ties. Also the top and bottom conductive layers preferably have similar electrical conductance and specific heat. In view thereof carbon or a carbon comprising material is very suited, such as carbon nanotubes with a specific heat of about 700 J/kgK; the characteristics of carbon nanotubes are found to at least partially resemble those of graphene . In an alternative approach nanoporous material may be coated with a 0.04-4 nm thick conducting layer, such as by ALD of a metal.

In an exemplary embodiment of the present method at least one of the bottom conducting layer 31 and top conducting layer 32 is provided with patterns. As such adjacent vertically aligned nanoporous material may be interconnected at a top side and/or at a bottom side; complex arrays may be formed in such a way; in addition the patterns for connection and openings 71 may partly or fully coincide.

In an exemplary embodiment the present method further comprises providing a conductive supporting layer 23 on the bottom conducting layer 31 (fig. 5a) . The thickness of this supporting layer is typically from 5-500 nm, preferably 10-250 nm, more preferably 20-100 nm, such as 30-50 nm; it may be of a metal or metal comprising material, such as TiN; it may be provided by sputtering or PVD; it may also be provided in a same equipment as the bottom conducting layer and optional sacrificial layer. It may further be provided, typically in a later stage, with a catalyst, such as a Fe comprising catalyst .

In an exemplary embodiment the present method further comprises providing a sacrificial layer 81 (fig. 5a) , optionally or typically on the conductive supporting layer 23. The thickness of this sacrificial layer is typically from 5-500 nm, preferably 10-250 nm, more preferably 20-100 nm, such as 30-50 nm; it may be of a metal or metal comprising material, such as Ti; it may be provided by sputtering or PVD; it may also be provided in a same equipment as the bottom conducting layer and supporting layer.

In an exemplary embodiment of the present method the substrate is provided with a back side dielectric layer 28 (fig. 5a) . The back side dielectric layer may have a similar thickness as the first dielectric layer 21 and may be provided in a similar or the same way, such as at the same time.

In an exemplary embodiment of the present method the second dielectric layer 22 is partly applies, such as by depo ¬ sition, on the first dielectric layer 21 (see e.g. fig. 5a, underneath 22) . In other words no bottom conducting layer is present and the combined dielectric layers may be considered to form a wall surrounding an area comprising the at least one receiving space and nanoporous material. A top conducting layer may still be provided at least partly on the second die- lectric layer, e.g. in order to provide contact to an outside world and/or to an adjacent thermoacoustic heater/sandwiched structure, if applicable.

In an exemplary embodiment of the present method the second dielectric layer 22 is provided with a thermally and chemically stable masking layer 83 (see fig. 5a) , such as SiC and SiN, for narrowing a gap between the nanoporous material and the second dielectric layer.

In an exemplary embodiment of the present method the nanoporous material is provided with at least one conformal layer, such as an dielectric layer, which may contribute to improved material properties and protection of the nanoporous material. The conformal layer may have a thickness of 0.1-20 nm.

In an exemplary embodiment of the present method the substrate is provided with a cavity (11) (fig. 3b) . The cavity may incorporate most or all of the further elements partly or fully. The cavity may have perpendicular walls, with respect to the substrate, or wall provided under an oblique angle, such as an angle of 10-170 (almost "closed"- almost

"open"=flat) degrees relative to the substrate.

In an exemplary embodiment of the present method at least one of the nanoporous material 41, openings 71, receiving space 51, top conducting layer 32, and bottom conducting layer 31 is patterned using lithography and/or e-beam. Typically various patterning steps are performed using alignment markers. In an example an I-line ASML PAS 5500 is used.

In a second aspect the present invention relates to a three dimensional thermoacoustic device according to claim 10. The present device has certain advantages such as that it is suit ¬ able for heterogeneous system integration, it is magnet free; it is free of vibration, it can be produced in a cleanroom, in high volume production, it requires no assembly of multiple parts, it is light in weight, it is scalable and can be miniaturized, it has a flat frequency response, and it may have a wideband response, such as from audio frequency to ultrasonic. The device may be considered to have a series of structures, e.g. the present heaters 200, which in turn may comprise a number of sandwiched structures; likewise, e.g. in view of a current path, the device may be considered to relate to a meandering structure; as such a length of the current path may be varied, and hence the electrical resistance R, and as a consequence the output (W) , such as by design, and therewith acoustical properties may be varied.

In an exemplary embodiment the present device is obtainable by the present method.

In an exemplary embodiment of the present device the na- noporous material has a ratio of surface area SA (m 2 ) to volume V (m 3 ) of > 10 4 irr 1 , preferably > 10 6 nr 1 . For CNT the ratio is found to be 9*10 6 nr 1 , based on calculations in combination with SEM measurements. The material is found to provide a tremendous device-to-environment interface. The 3D microstruc- tures made of the nanoporous material are found to be extremely suitable for surface driven energy transfer and signal exchange mechanisms with an outside medium, typically a fluid such as air. A similar ratio is a surface area per gram, as may be determined by a BET method (@77 K, N 2 ) , e.g. by using a Horiba SA-9600. For carbon about 5 m 2 /g has been found, whereas for CNT about 20 m 2 /g is found. Such is found to be very beneficial in view of thermoacoustic properties.

In an exemplary embodiment of the present device the nanoporous material is selected from carbon comprising material, such as carbon nanotubes (CNT) , and graphene . These materials are found to have good thermal and electrical characteristics and fit well within the present method and device.

In an exemplary embodiment of the present device at least one top electrical conductive elements 32 has at least one ar ¬ ray of symmetrical openings, typically most or all elements 32 have such an array of symmetrical openings. The array typi ¬ cally covers at least a part and often most of the underlying vertically aligned nanoporous material; wherein the conducting layer is in contact with the nanoporous material, and the openings are provided above the free space 61; the array comprises typically 10 ~6 -10 openings^m 2 , such as 10 ~5 -1 open- ings/μηα 2 , e.g. 10 ~4 -10 -2 openings/μπι 2 . A width of conducting lines surrounding said openings is typically from 1-200 μιτι, preferably from 2-100 μπι, more preferably from 5-50 μιτι, such as from 10-40 um, e.g. 20-30 μπι.

In an exemplary embodiment the present device bottom conductive pads of the bottom conductive layer are protected by a SiC layer from corrosion and lift-off during the HF vapor etching of oxide sacrificial layer, which results in good quality bottom metal layer.

Various details of the present invention can be found in the MSc thesis of one of the present inventors, "THERMALLY DRIVEN SOUND SOURCE : APPLICATION OF CNT NANOFOAMS" by Hengqian Yi, TU Delft, to be published December 2016, which document and its contents is herewith incorporated by reference.

The invention is further detailed by the accompanying figures and examples, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many var ¬ iants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims .

FIGURES

Figure 1: The development of sound source, starting from late 19th century till now very little improvements are made in the sound source industry, nowadays people still rely on the old inventions.

Figure 2: The working principle of a thermoacoustic chip and the energy transfer through Multiphysics domains.

Figure 3a-c: Illustration of the difference between 2D and 3D design, (a) Illustrates the 2D design and the less op ¬ timal orientation of acoustic emission. The 2D design is com- posed of a conductive thin film and an underneath cavity inside a substrate. A suspension feature is added in order to reduce thermal loss by conduction through the substrate. The orientation of a heat wave is limited by the geometrical nature of the 2D thin film design. The air expansion near the 2D thin film is in two directions: upward to the ambient which is desired, and downward into the cavity which is not desired. The suspension feature and thin film together give a more fragile property to such a 2D thin film design. (b)and (c) The present proposed 3D solution. Fig. 3b shows the principle of a 3D thermoacoustic design, wherein arrows show the air expansion orientation of such a 3D design. The present 3D solution is considered to benefit from vertical nanoporous heaters and the openings and cavities to direct the air expansion to the desired orientation. Fig. 3c shows the components of the pre- sent 3D design. The device can be wire bonded, to drive in an electrical current through wires, wherein a current input is 37 and an output is 38. The device can be fabricated in a cavity 11, or on a planar substrate 10 surface. Element 21 is the first dielectric layer, 28 is the backside dielectric layer, 31 is the bottom conducting layer, 23 is the supporting layer. Further 22 is the second dielectric layer and 32 is the top conducting layer. 41 is the nanoporous material and 42 is the vertical aligned nanoporous element. 61 is the freed space after removing 22 by etching. 71 is the top opening in 32 through which element 61 can be formed by etching.

Figure 4a-b: A 3D model of the present CNT nanofoam de- sign with hexagonal openings for sound emission, (a) Zig-Zag current created by vertically conducting CNT arrays, heating causes air expansion and result in sound emission from the openings. A current 37 enters on a left top side, passes through the top conducting layer, then through the nanoporous material, then trough the bottom conducting layer, the enters an adjacent (b) Side view of applied materials in the design.

Figure 5a-e: The schematic of main process steps and its correlating SEM images.

Figure 6: The frequency response of SPL of the 2D planar design and the 3D vertical design, (a) shows a simulation result of a 2D planar design. On the left the SPL response of the device at an input power of 0.5 W over the frequency range from 1 kHz to 100 kHz at 3 different distance, 2 cm, 3 cm and 5 cm is shown. The right part of (a) shows the distribution of the SPL field in all directions in the surrounding environment, (b) shows the simulation result under the same conditions as (a) with the only change being to the present3D design geometry. It shows that a 3D design has a more flat response over wide range of frequencies.

Figure 7a-c: The fabrication result of the present processing approach.

Figure 8: The SPL frequency response of thermoacoustic sound source (this work) . The SPL response is noisy as shown due to the low sound pressure detected is near the self-noise level of the Microphone.

DETAILED DESCRIPTION OF THE FIGURES

In the figures

100 three dimensional thermoacoustic device

200 3D thermoacoustic heater

300 sandwiched structure

10 substrate 11 substrate cavity

21 first dielectric layer

22 second dielectric layer

23 supporting layer

28 back side dielectric layer

31 bottom conducting layer/element

32 top conducting layer/element

37 electrical input

38 electrical output

41 vertically aligned nanoporous material

42 3D nanoporous material element

51 receiving space

61 freed space

71 etched opening

81 sacrificial layer

82 protective layer (for bottom conducting layer/element)

83 masking layer (on top of second dielectric layer)

84 Sealing layer (for top conducting layer/element )

90 electrical power supply

SA Surface area (m 2 )

V volume (m 3 )

The figures are further detailed in the description and examples below.

EXAMPLES/EXPERIMENTS

Thermoacoustic device

In an example of the present design, shown in fig. 3b and 3c, repetitive two CNT nanofoam comprising arrays are grown on an individual bottom contact pad with a gap in between said arrays, as shown in Figure 4. Thus the electrical connection between the two CNT arrays is bridged by the bottom metal pad. Between each two array units an electrical connection is achieved by a top metal layer. Hexagonal openings, designed for purpose of emitting pressure waves, are found to contrib ¬ ute to a uniform electrical current distribution and to provide mechanical robustness. High resolution and high efficiency lithography assisted patterning, such as I-line lithog- raphy, instead of e.g. laser patterning required in fabrication of CNT thin film based devices, is found to provide a large freedom on tailoring and adapting the present CNT nanofoam design into arbitrary formable shapes and the ability to achieve scalable micro/nano architectures.

Method of fabrication

The main process steps to construct the present CNT

nanofoam architecture is presented in Figure 5 along with SEM images for each step.

Despite that functional devices with excellent characteris- tics have been obtained some room for improvement is considered. Schematics of the main challenges and corresponding SEM images are shown in Figure 5. To solve some of these challenges the above process flow is further optimized by including several further steps. In Figure 5 the simplified flow chart concerning the main optimized steps are illustrated.

Method of CNT growth

The accurately patterned CNT growth within TEOS Oxide trenches mainly consists of 3 steps: catalyst deposition, lift-off and grow. A 0.1-10 nm, such as 5 nm, thin catalyst layer is applies, such as by deposition, on the wafer by evaporation, preferably comprising nanoparticles , such as Fe. The catalyst layer is preferably deposited on a conductive layer, such as a TiN or ZrN layer. On the catalyst, preferably on the nanoparticles, a well-controlled growth of nanoporous CNT is established. The photo-resist for masking the TEOS oxide is not removed after the dry-etching step. Thus during Fe deposition, an area of TEOS oxide is still covered by the photo-resist. With the lift-off process unwanted Fe is removed together with the photo-resist; only the TiN supporting layer at the bottom of trenches is applies, such as by deposition, with Fe. The lift-off process is carried out in an ultrasonic bath in NMP. In this way the TEOS area is prevented from Fe contamination and after lift-off they are considered as clean to any further CMOS process. The CNT growth rate which under condi ¬ tions of 5 nm Fe catalyst, 50 ran TiN supporting layer and 550 °C LPCVD method is around 1 um/min during about 10 min.;

therein a gas combination of N2, ¾ and C2H2 was used; the tool is an AIXTRON Black Magic CVD System. The respective gas amounts are N?:200 ppm, ¾:700 ppm, C2H2:50 ppm, the total pres ¬ sure is 80 mbar. This growth rate is within a desirable range for reaching a target height, the height of the nanotubes can be well controlled. The total CNT growth time is approximately 10 min to reach the same height as the TEOS.

Results

To estimate the performance of the present design, inven ¬ tors simulated a comparable 2D planar device and the present 3D vertical design. The two designs were simulated (using COM- SOL) using the same material and physical boundary conditions for comparing the differences in their performance. In view of 2D and 3D designs the only change between the 2 models required in COMSOL was the geometry. The parameters regarding materials and thermoacoustic and solid interaction physics are the same used in the two comparative models.

The frequency response of sound pressure level (SPL) from 1 kHz to 100 kHz is plotted in Figure 6a-b. The curves (bottom) show a much more flat frequency response of the present 3D vertical design (in comparison to the 2D planar device (top) ) over the whole spectrum that is simulated. The value of sound pressure level increased largely from low frequency to high frequency in the case of CNT thin film planar thermoacoustic sound source. And in the present design of CNT nanofoam vertical volume heater the SPL value is found to be more constant from low to high frequencies. It is noted that a smooth and flat response can result in better reproduction of the real sound and as such may be considered to be a requirement.

The present fabricated CNT nanofoam thermoacoustic (TA) device is shown in Figure 7a-c. The chip therein is wire bonded to a PCB for acoustic measurement. For fabricating the device, it may require only three layers of masks and each device may be fitted to a 6 x 6mm die on a 100 mm wafer. Some test structures are fabricated along the device as well. In fig. 7b a number of sandwiched elements 300 is shown, as well as a ther- moacoustic heater 200.

With an input power of 32mW, the present TA chip produces distinguishable sound. The measurement result of the SPL spectrum of the CNT nanofoam device is given in Figure 8.

The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying examples and figures.